Ophthalmic lenses have conventionally been prepared from glass compositions exhibiting a refractive index of 1.523. Lenses fashioned from such glasses having a high negative power are cosmetically unappealing, however, because of their very thick outside edges. The use of glasses with a higher index of refraction permits an increase in the radius of curvature of a lens and, as a consequence, a reduction in thickness of the lenses. Nevertheless, glasses with higher values of refractive index have found but limited use until recently, because the high index glasses previously available were prepared from compositions wherein the density increased so rapidly with increasing refractive index that the use of such glasses led to heavier lenses, resulting in discomfort to the wearer. Furthermore, an increase in refractive index frequently effects a substantial increase in the dispersion of the glass.
A number of new glass compositions containing large amounts of TiO.sub.2 have been devised which yield a desirable combination of high refractive index and moderately low density. Such glasses have resulted in a dramatic increase in the use of high index glasses in the fabrication of ophthalmic lenses. The success of those glass compositions in the ophthalmic art has led to considerable research to develop photochromic glasses exhibiting refractive indices of at least 1.6.
By far the greatest number of commercially marketed, ophthalmic photochromic lenses have been prepared from silver halide crystal-containing, borosilicate-based glass compositions. Unfortunately, one cannot make a photochromic glass by merely adding silver and halides to any arbitrarily chosen recipe for producing a high index glass. For example, it is difficult to precipitate silver halides in a transparent borosilicate glass unless the composition contains a relatively high concentration of B.sub.2 O.sub.3 and unless the ratio of modifiers to B.sub.2 O.sub.3 is in the proper range. Hence, any of the materials which can alter the refractive index may influence this ratio and, consequently, the ease with which the silver halide can be precipitated.
Previous research has demonstrated that substantial quantities of Ta.sub.2 O.sub.5,HfO.sub.2, Nb.sub.2 O.sub.5 and ZrO.sub.2 can be incorporated in the host glass composition to raise the refractive index without significant deleterious effect upon the photochromic behavior or the chemical durability of the glass. However, Ta.sub.2 O.sub.5 and HfO.sub.2 are not only exceedingly expensive, but also they raise the density of the glass significantly. Nb.sub.2 O.sub.5 and ZrO.sub.2 give rise to melting problems when included in high concentrations. If TiO.sub.2 imparted no adverse effects to a photochromic glass, one could simply add enough TiO.sub.2 to achieve the desired refractive index, because that approach would yield the lowest possible glass density compatible with any specified refractive index. Unfortunately, however, the rise in refractive index brought about through TiO.sub.2 additions is accompanied with a yellow-brown color and a low Abbe number (a high level of dispersion). Moreover, too high concentrations of TiO.sub.2 generate opacity in the presence of silver halides and also lead to a reduction in the darkening capability and fading rate of the glass.
La.sub.2 O.sub.3 raises the refractive index of the glass, but hazards the development of phase separation therein and increases the density thereof. Alkaline earth metal oxides can be utilized to raise the refractive index of a glass, but the inclusion of substantial amounts has a detrimental effect upon the photochromic properties thereof. Lead appears to be able to enter the silver halide crystal and to depress the rate of fading dramatically, besides markedly increasing the density of the glass.
With those constraints in mind, the principal objective of the present invention was to develop glass compositions suitable for ophthalmic applications exhibiting good photochromic properties and a high index of refraction, but coupled with a relatively low density and high Abbe number.
A specific objective was to develop such glass compositions which would demonstrate a refractive index (n.sub.d) of greater than 1.59, a density (.rho.) no higher than 3.1 g/cm.sup.3, and an Abbe number (.gamma..sub.d) of at least 40; which darken to a transmittance below 40% when exposed to actinic radiation under standard solar simulation at room temperature and fade at least 15 percentage points of transmittance within five minutes after withdrawal from the actinic radiation provided by the solar simulation, and which are subject to a weight loss no greater than 0.01 mg/cm.sup.2 of surface area when exposed to the standard American Optical chemical durability test.
A preferred objective was to develop such compositions which could be chemically strengthened through a conventional ion exchange treatment involving the exchange of larger alkali metal ions from an external source with small alkali metal ions in the glass surface, the exchange being carried out at temperatures no higher than the strain point of the glass.